1. Field of the Invention
The present invention relates to optical interleavers for combining and separating pluralities of optical wavelength channels. More particularly, this invention pertains to a Mach Zender interleaver.
2. Description of the Prior Art
The current and projected growth of the Internet has led to readily foreseeable demands for increased bandwith to service both domestic and foreign consumers and businesses. An accepted method for addressing the bandwidth and cost demands of rapidly-proliferating networks and connections has been the integration of optical elements into such networks.
Optical fiber, due to its relatively high bandwidth and low cost is the preferred means of transmission of voice and data at rates greater than a few tens of megabits per second and over a kilometer or more.
In view of vast increases in projected demand, future network development must take advantage of devices and arrangements that are capable of enhancing traffic many times over without commensurate increases in materials (e.g. optical fiber) and other costs.
Cost per bit per mile (cost of transporting data traffic to a user) is a critical measure of cost effectiveness in communications networks. This may be improved by increasing transmission distance (through the use of such technologies as Raman amplification or amplification based on Erbium-doped fibers) or the number of bits carried (e.g., by employing higher bit rates in a single wavelength in addition to using time division multiplexing.
Another approach to enhancing the cost effectiveness of network designs relies upon the improvement of component performance to increase the number of wavelength channels. The extension of the number of wavelength channels, while maintaining channel spacing, requires the development of new amplifier designs due to the need to increase wavelength range. Further bandwidth enhancement may be obtained through the development of devices compatible with reduced channel spacing.
Reductions in channel spacing place increased performance requirements upon optical filters. Currently, major filtering technologies include thin-film filters, arrayed waveguide gratings and fiber Bragg gratings. Each faces technological challenges in adapting to reduced channel spacings. Thin-film filters, while satisfactory for 400 and 200 GHz dense wavelength division multiplexing (DWDM) systems, are difficult to adapt to channel spacings of 100 and 50 GHz with acceptable yields. A large number of Bragg gratings is required for narrow filter passbands since a single device is required to separate one wavelength. Thus, scaling to a high channel count requires numerous devices. Further, while they are easily coupled to a fiber, expensive circulators are required as they reflect the filtered wavelength to the input fiber.
Arrayed waveguide gratings (AWG), currently commercially available for 40 channels with 100 GHz spacings, can readily separate a spectrum of wavelengths into individual channels. Design and manufacturing tolerances complicate the manufacture of arrayed waveguide gratings as channel spacing decreases. The cost and insertion loss of AWG""s with more than 40 channels are extremely high.
The interleaver is a device that combines two input sets of wavelengths in which the channels of one set of wavelengths are offset by one half the channel spacing from those of the other set. Such a device is ideal for ultra dense networks. Further, interleavers can work in reverse to separate a single densely packed channel set into two output fibers, each of twice the channel spacing of the original set. Interleavers may be cascaded to provide further channel separation on four output fibers, each transmitting one fourth of the number of channels and four times the channel spacing. An interleaver or an array of interleavers allows the use of simpler thin-film filters or arrayed waveguide gratings to separate the individual channels.
Various interleaver configurations have been proposed including liquid crystals, birefringent crystals and others. Interleavers based upon a fused-fiber Mach Zender interferometer offer a simple, cost-effective design. However, careful control of the fiber path length difference between the two arms of the interferometer is essential to obtain the correct channel spacing for matching the device to the ITU (International Telecommunications Union) grid.
The present invention overcomes the foregoing shortcomings of the prior art by providing, in a first aspect, an optical interleaver. Such interleaver includes a first coupler and a second coupler joined to one another by arms of optical fiber. One arm of the interleaver includes an optical image transfer element intermediate the ends of fiber segments from the first and second couplers.
The foregoing and additional features and advantages of this invention will become further apparent from the detailed description that follows. Such description is accompanied by a set of drawing figures. Numerals of the drawing figures, corresponding to those of the written description, point to the features of the invention with like numerals referring to like features throughout both the written description and the drawings.